This invention relates to full color, dot next to dot, xerographic printing systems or apparatus, and more particularly to a tandem tri-level xerographic apparatus and method for producing quality pictorial color images.
Xerocolography (a form of xerography for dry color printing) is a color printing architecture which combines multi-color xerographic development with multiwavelength laser diode light sources, with a one polygon, single optics ROS and with a polychromatic, multilayered photoreceptor to provide color printing in either a single pass or in two passes. Inherently perfect registration is achieved since the various color images are all written at the same imaging station with the same ROS. In all three latent images are written in this manner. Two of the three images are immediately developable because their voltage levels are offset from a background level while the voltage level of the third image is at the time of its formation equal to the background voltage level. Before the third image can be developed, the photoreceptor must be exposed to flood illumination of a predetermined wavelength.
Xerography is capable of producing either highlight color or process color images in a single pass as well as in multiple passes. In creating full process color images, using Image-On-Image (IOI) technology, toner particles are deposited on already developed toner images. With this type of imaging, it is desirable to use Non-interactive Development (NID) in order to avoid scavenging of an already developed image.
Conventionally, full gamut color imaging in a single pass is possible using four or more voltage level images but these systems suffer from the need to form latent images by exposing through already developed images. As evidenced by the success of the commercially available highlight color machines which use tri-level imaging, the development fields which are half those of conventional xerography are practical. However, four or more voltage level images are difficult to develop because of the problems of dealing with large cleaning fields and small development fields.
In a conventional tandem architecture, four separate xerographic engines, each consisting of a ROS, a photoreceptor and a development system are used in series to develop and transfer the CMYK toners needed to produce process color images. If a special color is needed for a logo or to broaden the color gamut it must be added as a fifth xerographic engine with ROS, photoreceptor and development system. Known tandem engine imaging apparatuses require as many as four separate image registrations. As will be appreciated, the more image registrations required the more there is a possibility for image misregistration resulting in unwanted color overlapping and fringing.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, are intended to provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 5,221,954 entitled "Single Pass Full Color Printing System Using A Quad-Level Xerographic Unit" granted to Ellis D. Harris on Jun. 22, 1993 discloses a four color toner single pass color printing system consisting generally of a raster output scanner (ROS) optical system and a quad-level xerographic unit and a tri-level xerographic unit in tandem. The resulting color printing system would be able to produce pixels of black and white and all six primary colors. The color printing system uses a black toner and toners of the three subtractive primary colors or just toners of the three subtractive primary colors.
U.S. Pat. No. 5,223,906 entitled "Four Color Toner Single Pass Color Printing System Using Two Tri-Level Xerographic Units" granted to Ellis D. Harris on Jun. 29, 1993 discloses a four color toner single pass color printing system consisting generally of a raster output scanner (ROS) optical system and two tri-level xerographic units in tandem. Only two of the three subtractive primary colors of cyan, magenta and yellow are available for toner dot upon toner dot to combine to produce the additive primary colors. The resulting color printing system would be able to produce pixels of black and white and five of the six primary colors, with pixel next to pixel printing producing all but the strongest saturation of the sixth primary color, an additive primary color. The color printing system uses either four color toners or a black toner and three color toners.
U.S. Pat. No. 5,337,136 entitled "Tandem Tri-level Process Color Printer" granted to John F. Knapp et al on Aug. 9, 1994 discloses a tandem tri-level architecture. Three tri-level engines are arranged in a tandem configuration. Each engine uses one of the three primary colors plus one other color. Spot by spot, two color tri-level images can be created by each of the engines. The spot by spot images are transferred to an intermediate belt member, either in a spot on spot manner for forming full color images or in a spot next to spot manner to form highlight and/or logo color images. The images created by the tri-level engines can also be transferred to the intermediate in a manner such that both spot next to spot and spot on spot transfer is effected.
Previous or conventional tri-level Xerographic processes typically produce registered, two color or high light color images at within a range of about 50 to 90 prints per minute. As disclosed above, the intriguing possibility of making full color images in a single pass by overprinting or superimposing two tri-level images has occurred to others. Their ideas generally take the form of either a two cycle machine or two tri-level processes in series or tandem along one continuous belt photoconductor, with each tri-level process having a separate ROS. The throughput rate for the single pass version is the same as the tri-level process itself, while throughput rate for the two cycle arrangement is half or less. Unfortunately, neither of these approaches is capable of producing a full color gamut because the two colors that make up the composite tri-level image on a single imaging module can never be superimposed, i.e., they are mutually exclusive. For example, if a tri-level image is printed using colors A & B on a single imaging module, which is then superimposed over a second tri-level image printed with colors C & D, it is possible to obtain the colors A+C, A+D, B+C, and B+D in addition to colors A, B, C and D developed one next to the other. However, it is not possible to obtain superimpositions of A+B or C+D. In this case, if ABCD represented KYMC, it would not be possible to print blue (M+C or C+D) or overprint yellow on black (K+Y or A+B).
Moreover, unless the wave length of the exposure unit used were such that the second tri-level latent image can be exposed through the pre-existing first tri-level developed image, then the above process requires that the two composite tri-level images be developed and transferred separately. This of course is not true if the two tri-level images are developed and registered "side by side" using the color set KRGB instead of that KYMC. However, if this is to be accomplished using one transfer, the second tri-level image separation must be developed with a non-contact, cloud development system which does not respond to the gradients or to the large reverse (cleaning) field associated with the companion color latent image. Unfortunately however, there is no known development system that satisfies these requirements.
Current conventional approaches to full gamut color printing include the tandem engine approach, and the multiple superposition REaD (Recharge, Expose and Develop) approach. Both can be implemented in a cyclic mode with as few as one ROS. Although a multi-cycle color process uses fewer hardware components (one charge, ROS, and cleaning station), its throughput rate is ordinarily less than, or equal to, the process speed divided by the number of process cycles. Furthermore, in the cyclic mode, each development system, and the cleaning system, (and in the case of REaD, the transfer station), must be enabled and disabled every print cycle. In addition, at least 4 color separations must be registered.
One pass REaD requires a single, long photoreceptor to accommodate four or more recharge, expose and development stations. The manufacturing yield on long, defect free, belt PCs is very low at present. Photoconductors of the length required for REaD also cause tracking/registration problems and are difficult to replace in the field.
There is therefore a need for a relatively simpler YKMC system in which image portions or spots can be printed, not only in YKMC, but also in Y+K superimposed, and M+C or in general with the two colors on any imaging module superimposed, thereby extending the color gamut and achieving pictorial quality final images.